1. Field of the Invention
The present invention generally relates to a nozzle for an atomizer which produces a jet of liquid in the form of a mist and, more particularly, to a nozzle assembly applicable to an ultrafine particle atomizer of a type which produces an extrafine mist of liquid, such as water, fuel oil, or medical solution, having a mean particle diameter (a Sauter mean particle diameter as referred to hereinafter) ranging from a submicron to some ten microns as most, or in other words, a dry mist which does not feel wet if touched (referred to hereinafter as an "ultrafine mist").
2. Description of the Related Art
Atomizers are employed in various fields for various purposes, such as humidifying, cooling, dust controlling, disinfectant solution spraying, and fuel oil atomizing. Generally, it is desirable that any mist produced by means of such a device should be an ultrafine mist. The reason is that of component particles of the mist are coarse, the surfaces of circumjacent objects will get wet in a given period of time when, for example, the atomizer is employed for humidifying purposes; and if the atomizer is employed for the purpose of disinfectant solution spraying, the circumjacent objects will get wet resulting in stains being left thereon.
The present inventor, after his series of studies on such a problem, found that for an ultrafine mist to be realized its component liquid particles must not have a maximum particle diameter greater than 50 microns and not have a Sauter mean diameter greater than 10 microns. On the basis of such a finding, the present inventor has already proposed various ultrafine mist producing atomizers (Japanese Published Unexamined Patent Application Nos. 54-111117, 55-49162, and 57-42362).
There are two types of nozzle assemblies, one or the other of which is employed in the ultrafine mist producing atomizers proposed by the present inventor. One type involves passing compressed air through a passage outside the nozzle tip, which may be called the outer air-passage type (Japanese Published Unexamined Patent Application Nos. 55-49162 and 57-42362). The other type involves passing compressed air through a passage defined within the nozzle tip, which may be called the inner air-passage type (Japanese Published Unexamined Patent Application No. 54-111117). From the standpoint of preventing the diffusion of a jet stream of a gas-liquid mixture from the nozzle orifice, it is generally believed that nozzles of the outer air-passage type are preferable.
As an illustration of a nozzle of the outer air-passage type, a general arrangement of the nozzle in the ultrafine mist producing atomizer disclosed in said patent publication No. 55-49162 is described below by way of example.
The basic arrangement of this nozzle is generally identical with that shown in FIGS. 1 and 2, on which one embodiment of the present invention is based. That is, a nozzle body has a plurality of nozzle heads arranged in an equi-spaced relation around the longitudinal axis thereof, each of the nozzle heads having a mounting hole in which a nozzle tip is mounted. Each nozzle tip, as can be seen from FIG. 12 (in which a part of a nozzle is shown), has a liquid passage hole 5a, while an air jet passage 5e is defined in a mounting hole 5b between a nozzle body 5c and the outer periphery of a nozzle tip 5d. Individual mounting holes and individual nozzle tips are so arranged that the respective longitudinal axes of the nozzle tips converge at one point on the longitudinal axis of the nozzle body, whereby as currents of compressed air are caused to jet out toward said one point on the longitudinal axis of the nozzle body passing, through the air jet passages, the currents suck liquid thereinto through the respective front end openings 5f of the liquid passage holes to form jet streams of a gas-liquid mixture and the jet streams impinge against one another at said one point on said longitudinal axis, thereby producing an ultrafine mist of liquid.
With respect to the above-described prior art nozzle arrangement, it must be noted that, as FIG. 12 shows, the front end openings 5f of the liquid passage hole 5a defined in each nozzle tip 5e are open at sides of the front end 5g of the tip and not on the front end 5g itself; that the angle of taper of a front end tapered portion 5h of the nozzle tip 5d is about 7.degree.-22.degree.; and that the front end of the nozzle tip 5d projects little, if any, from the nozzle body 5c (the amount of such projection being in the order of 0.2 mm at most).
Now, in the prior art nozzle arrangement, the relationship between compressed air pressure and liquid atomization rate is shown in FIG. 4a (conditions in FIG. 4 are: liquid pressure=0; liquid suction height=100 mm). In other words, there is no proportional relationship between compressed-air pressure and liquid atomization rate. In FIG. 4a, the mean particle diameter in the mist is about 50 microns--about 10 microns in a low pressure zone ranging from an initial air pressure at which atomization starts to a pressure level of about 3 kg/cm.sup.2 with no ultrafine mist being available realized. An ultrafine mist having a mean particle diameter of less than about 10 microns is produced only in a high pressure zone in which the air pressure is in excess of about 3 kg/cm.sup.2. However, as air pressure becomes higher, the mean particle diameter becomes smaller, and as shown in FIG. 4a, atomization is terminated when an air pressure of less than 4 kg/cm.sup.2 is reached. With prior art arrangement, therefore, one problem is that at on/off control stages for compressed air supply, a mist having a relatively coarse particle size is produced, so that the floor and circumjacent surfaces get wet. Another problem is that when only a small amount of ultrafine mist is required, it is necessary to increase the air pressure, which means a disproportionally greater amount of air consumption for the liquid atomization is required which is extremely uneconomical. A further problem is that the diameter of particles in the mist varies with changes in the air pressure, or in other words, a mist having a constant particle diameter cannot be produced.
These problems are considered to be attributable to the front end structure of the nozzle and, more particularly, to the fact that a negative pressure develops thereat as a compressed air current passes at a supersonic velocity through the nozzle orifice.